1. Field of the Invention
The invention relates generally to a laser machining head for machining a workpiece by means of a laser beam so that welding tasks, cutting tasks, etc can be carried out on the workpiece, and more particularly, to a laser machining head whose position in relation to a workpiece may be controlled easily.
2. Discussion of Background Art
It is already generally known to measure the distance between a workpiece and a laser machining head during the machining of the workpiece by a capacitance method, that is to say using a sensor electrode electrically connected to the laser machining head, which supplies a sensor signal corresponding to the distance whose magnitude depends on the capacitance between the sensor electrode and the workpiece. The sensor electrode may be the electrically conducting tip of a nozzle from which the laser beam emerges.
By monitoring the sensor signal it is possible to position the laser machining head relative to the workpiece in order that the workpiece can be machined in a suitable manner. Positioning is done in this case by means of a control device which receives the measured distance as an actual value and controls the position of the sensor electrode or of the laser machining head as a function of a comparison of the actual value with a specified desired value.
In this distance adjustment, however, it is not necessary to constantly to displace the entire laser machining head. On the contrary, it suffices to change only the position of a section of the laser machining head facing towards the workpiece, this section carrying a lens optics system which serves to focus the laser beam. Due to the positional adjustment of this section the position of the focus of the laser beam relative to the workpiece can then be kept constant, which is necessary for the faultless machining of the workpiece.
DE 196 22 413 C2 already discloses a laser machining head for machining a workpiece by means of a laser beam. This known laser machining head contains a head housing through which the laser beam passes along a central axis; a carrier member for a lens optics system for focusing the laser beam mounted on the head housing and displaceable relative to the latter in the direction of the central axis; a first setting device to which a cable connected to the carrier member and a cable drum connected to the head housing for winding up the cable belong in order to move the carrier member towards the head housing; and a second setting device by means of which the carrier member is displaceable in the direction away from the head housing.
In the known laser machining head the carrier member is composed of a kind of cage with end rings located opposite one another between which a cable is stretched which is wound around a cable drum. Thus, depending on the direction of rotation of the cable drum the carrier member can be moved in one direction or another. Since the cable drum comes to lie between the end rings of the carrier member, the latter has a relatively great overall length which results in a relatively long and hence heavy laser machining head, since on account of its size the carrier member is also relatively heavy.
The underlying aim of the invention is to refine a laser machining head of the type described above in such a way that it has a small overall length and a lower weight.
A laser machining head according to the invention distinguishes itself in that the second setting device is separate from the first setting device. That is, it is arranged or constructed spatially separated from the latter. The overall length of the carrier member can, therefore, be considerably reduced compared to the overall length of the known carrier member, which results in a shorter laser machining head in the longitudinal direction of the beam and in a considerable reduction in its weight. Thus, a laser machining head of this type can be employed more universally and handled more easily.
According to a development of the invention, the second setting device is constructed as a compression spring which is arranged between the head housing and the carrier member in such a way that it is supported at one end on the head housing and at the other end on the carrier member and seeks to move the carrier member away from the head housing. The displacement of the carrier member relative to the head housing along the central axis can then ensue when, on the one hand, the cable is wound up against the force of the compression spring in order to pull the carrier member towards the head housing or when, on the other hand, the brake of the cable drum is released so that under the action of the compression spring the cable is wound off the cable drum, as a result of which the carrier member is moved away from the head housing. Its final position is fixed, in that the rotation of the cable drum is monitored and stopped at a time corresponding to the final position of the carrier member. The cable drum would then have moved through a corresponding angular path which is determined with the aid of a rotary or angle decoder coupled to the shaft of the cable drum. On winding up the cable onto the cable drum against the force of the compression spring, a corresponding procedure can be used to determine the final position of the carrier member. Here also the cable drum is then stopped again after a predetermined angular path detected by the angle decoder has been covered. In other words, a change in the distance between the laser machining head and the workpiece could be converted here into angular paths in order to control the rotation of the cable drum.
According to another development of the invention the second setting device is a hydraulic device for the hydraulic displacement of the carrier member. In this case, between the head housing and the carrier member a hydraulic cylinder, for instance, may be used through whose piston the carrier member is displaceable. If the chamber of the hydraulic cylinder is filled with hydraulic liquid while the cable drum is simultaneously released the carrier member moves away from the head housing, while in the reverse direction of motion with the winding of the cable onto the cable drum, the hydraulic liquid can flow out of the hydraulic cylinder. The displacement position of the carrier member relative to the head housing can once again be determined in both cases through the angular path covered by the cable drum.
According to another and highly advantageous development of the invention the head housing and the carrier member act as a setting device and can form a hermetically sealed inner chamber to which an excess pressure can be applied for the pneumatic displacement of the carrier member. A relatively high excess pressure in the inner chamber when the brake of the cable drum is released then results in the displacement of the carrier member away from the head housing, wherein here also the rotary position of the cable drum may again be monitored, using the aforesaid angle decoder for instance, to determine the final position of the carrier member. If, on the other hand, the carrier member is to be moved towards the head housing the cable drum is again set in operation in order to wind up the cable. In doing so, any further excess pressure building up in the inner chamber can be released through a blow-off valve so that the drive of the cable drum in this case does not have to work against an excessively high counteracting force. In the specified example a relatively high excess pressure would constantly prevail in the inner chamber.
In order to prevent this, a suitable excess pressure in the inner chamber for displacing the carrier member can be provided but only for the time needed to displace the carrier member along a predetermined path. On reaching the displacement position of the carrier member the excess pressure can then be reduced immediately to a relatively low value so that no constituents from the outside can penetrate into the inner chamber and thereby damage devices by soiling them. The drive of the cable drum can operate without difficulty against this relatively low excess pressure when the carrier member is to be moved again towards the head housing.
Preferably according to a development of the invention the cable, the cable drum and a drive provided for them are arranged in the inner chamber so that they are better protected against environmental effects.